In recent years, the following voice or speech synthesis techniques have been proposed. Such proposed voice or speech synthesis techniques include a technique that synthesizes and outputs voice corresponding to a speaking tone and voice quality of a user and thereby generates voice more like real human voice (see, for example, Patent Literature 1), and a technique that analyzes voice of a user to evaluate psychological and health conditions etc. of the user (see, for example, Patent Literature 2). Also proposed in recent years is a voice interaction system which implements voice interaction with a user by outputting, in synthesized voice, content designated by a scenario while recognizing voice input by the user (see, for example, Patent Literature 3).    Patent Literature 1: Japanese Patent Application Laid-open Publication No. 2003-271194    Patent Literature 2: Japanese Patent NO. 4495907    Patent Literature 3: Japanese Patent NO. 4832097
Let's now assume an interaction system which combines the aforementioned speech synthesis technique and the voice interaction system, and which outputs, to a question given by voice of a user, a response in voice synthesized through voice or speech synthesis. Such an assumed interaction system, however, may sometimes give the user a feeling as if a machine were speaking the response.